Polystyrene

Polystyrene is amorphous, rigid, and glass-clear in its general-purpose (GPPS) form. It is the lowest-cost clear rigid resin available and molds to very tight tolerances.

It is also brittle and chemically vulnerable. Those two limits define where it can be used. PS is a correct specification for a narrow set of applications and an expensive mistake outside them.

• Clarity: Excellent in GPPS. Water-clear.
• Stiffness and dimensional stability: Very high. Holds tolerances better than any polyolefin.
• Shrinkage: Low and predictable at 0.4-0.7%, which suits tight-tolerance jar threads.
• Impact: Poor. It cracks rather than deforms.
• Chemical resistance: Poor. This is the limiting factor.

Small cosmetic and sample jars for water-based formulas. Cream jars where the formula is aqueous and non-oily. Display and retail packaging.

Process: Injection molding.

Shrinkage: 0.4-0.7%.

Do not specify PS for terpene-bearing cannabis products, essential-oil formulas, or alcohol-based fills. Polystyrene crazes and cracks on contact with terpenes, limonene, and most essential oils. This has failed in the field on cannabis concentrate jars specified on unit cost alone. If the fill contains oils or terpenes, specify PP, PETG, or SAN instead.

Watch for: Stress cracking and crazing radiating from gates. Keep gate land short and avoid packing pressure spikes.